Method of hot topping using vermicular graphite



United States Patent Ofiice fiddfild Patented Mar. 14, 1S6? Mich, assignc-rs to The Dow Chemical Company, Midland, Mich a corporation of Delaware N Drawing. Fiicd Jan. 7, 1965, Ser. No. 424,136 4 Claims. (Cl. 22-212) This invention relates to a method of hot topping cast metal, and more particularly to a method of hot topping risers employed in metal casting.

One of the more significant problems encountered in the casting of molten metals is that'of controlling the cooling rate of the cast metal. As the cast metal cools, shrinkage usually takes place. When cooling is uneven, at least one portion of the casting solidifies while at least one other portion of the casting is still molten and shrinking at a greater rate. Uneven cooling causes undesirable properties such as stresses, voids, or the like in the finished casting and is to be avoided. Usually, most rapid cooling takes place where the casting is exposed to the air, generally at the top or mouth of the mold, It is, therefore, advantageous to insulate such exposed portions of the casting from the air, thereby to obtain more uniform cooling. Such insulation is referred to herein as hot topping.

Because of the shrinkage of molten metal as it cools and solidifies, many molds are fitted with risers. Risers are channels or conduits, usually having at least a portion of their length extending upwardly from the horizontal top of the mold. As metal is cast into the mold, the mold fills and metal runs into the risers. On cooling, as the cast metal in the mold shrinks, metal is drawn from the risers to make up the shrinkage loss, thereby permitting obtention of castings having dimensions true to the original mold. However, if the metal in the risers is allowed to cool prior to the metal in the body of the mold, the advantages of the risers may be substantially lost. Premature cooling of the riser metal may prevent the metal from being drawn into the mold to make up for shrinkage loss therein. Thus, the risers usually also are hot-topped so as to obtain a substantially uniform cooling rate throughout the cast metal.

In general foundry practice, risers are hot-topped by a riser compound which ordinarily consists of a refractory, particulate material. An effective riser com pound is a material which, upon application to the top surface of the riser, prevents the riser surface from freezing over during the time interval that the casting is cooling and shrinking. Preferably, riser compounds should be easy to handle, clean and store, vermin proof, nonmoisture absorbent and should produce little or no smoke or fuming during use.

In the past, relatively dense carbonaceous materials have been used as riser compounds in iron and steel casting. Such materials include graphite powders (usually amorphous graphite), mixtures of graphite powder plus refractory sands, mixtures or graphite powder plus carbon powders, and thermally degradable organic materials such as rice hulls and the like.

We have unexpectedly discovered that expandable graphite flakes perform excellently as a superior riser compound and insulating hot-top for molten metal and have all the preferred characteristics listed hreinbefore required for a riser compound. Furthermore, an unexpected advantage of the present invention is that the apparent low bulk density (approximately 0.5 to 5 lbs./ft. of the final expanded graphite coupled with its low thermal conductivity values (about 1 to 4 B.t.u.s/hour/ft. F./inch) yields a superior heat shield while simultaneously markedly reducing the mass of material used over that required with conventional hot-topping materials. Typical applications of expandable graphite require only about /3 to about of the amount of other carbonaceous riser compounds used conventionally.

The use of expandable graphite as a riser compound has still an additional advantage in the metal casting industry. The low mass of actual carbon in contact with molten metal in the riser provides for a much lower amount of carbon pickup in the riser metal than with the use of conventional carbonaceous riser materials. Thus the costly steps of decarbonizing remelted cast metal can be eliminated if expandable graphite is used as the riser compound in accordance with the present invention.

These and other objects and advantages of the present invention will become apparent from the following specification and claims.

In accordance with the present invention, exposed portions of molten metal in a melting pot, ladle or cast in a mold while at a temperature of at least about 150 C., and ordinarily at temperatures of at least about 500 C., are covered with a layer of acid-treated graphite which is capable of undergoing expansion at this tem perature.

The term acid-treated graphite as used herein includes both graphite in the heat expandable form and graphite which has been heat expanded prior to its introduction onto a molten metal surface.

Expandable graphite, in an amount of from about 1 to about 10 or more grams per square inch (gm/m and preferably from about 1 to about 5 gm./in. of molten metal surface is placed on the metal surface.

In operation, as the expandable graphite contacts the molten metal, it expands to a volume of from about 20 to about 600 times its original volume and usually expands to a volume of from about 200 to about 600 times its original volume. The resultant low density expanded vermicular graphite acts as an excellent insulator to assure substantially uniform cooling of the molten metal, as in a mold to prevent unduly rapid cooling of a molten metal in a ladle or holding pot. An unexpected advantage of this unique hot-topping process is that the graphite, as it expands, also conforms generally to the shape of the top of the mold, riser or pot or ladle thereby eliminating the need for cutting or otherwise forming the hot-topping material to a predetermined shape.

The graphite used in the present invention is prepared from particulate naturally occurring crystalline flake graphite or crystalline lump graphite, flake graphite being preferred. The crystalline graphite is given a particular acid treatment thereby rendering said graphite into the expandable state. The particle size of graphite to be used is not critical although ordinarily particles of from about 10 to about 325 mesh US. Standard Sieve are used.

Ordinarily in preparing the acid-treated graphite, a particulate natural crystalline graphite is contacted at about room temperature with (l) a mixture of from about 8 to about 98 weight percent concentrated sulfuric acid (at least about 90 weight percent H and from about 92 to about 2 weight percent concentrated nitric acid (at least about 60 weight percent HNO or (2) fuming nitric acid, or (3) fuming sulfuric acid, or (4) concentrated sulfuric acid (at least about Weight percent H SO or concentrated nitric acid (at least about 60 weight percent HNO plus at least about 2 weight percent of a solid inorganic oxidizer such as, for example manganese dioxide, potassium permanganate, chromium trioxide, potassium chlorate and the like. The resulting mix components usually are employed on a weight proportion basis of from about 0.2-2/ 1 (acid member/graphite). These are maintained in contact for at least about one minute although a lengthy contact time of hours or days is not detrimental. The acid treated graphite now expandable, is separated from many excess acid, washed and dried, if desired.

If already expanded grahite is to be used in the instant novel invention the acidified graphite is heated until exfoliation or expansion occurs. The preferred method of heating is to contact the acidified graphite with a hydrocar-bon flame.

Alternatively, another method or" preparing the expandable graphite used in the method of the present invention is to treat t e graphite material with an aqueous peroxy-halo acid, preferably perchloric or periodic acid, using an acid concentration of from about 2 to about '70 weight percent or more and an acid/graphite Weight proportion of from about 0.5-2/1. The acid treated graphite, now expandable, is separated from excess acid, washed and dried if desired and can be heated to give the expanded feed stock.

The crystalline graphite also can be anodically electrolyzed in an aqueous acidic or aqueous salt electrolyte at an electrolyte temperature of from about to about 80 C. at a mini-mum cell potential of about 2 volts. The total quantity of electricity passed is equivalent to from about It) to about 500 ampere-hours per pound of graphite. Electrically treated graphite, now expandable, is separated from the electrolyte solution.

As indicated hereinbefore, the resulting acid-treated graphite product can be expanded in situ when placed in contact with molten metal surface which is at the indicated temperature or can be otherwise heat expanded and then incorporated onto the molten metal surface. If the graphite is expanded prior to its placement on a molten metal surface, it can be further treated and formed by a slight compression and shaping for example, into an integral unit of predetermined shape.

The term apparent bulk density as used herein is the density determined from the'volume occupied by a given mass of the product subjected to free fall (by gravity) into an open top container, for example a graduated cylinder.

Metals which may be cast in accordance with the present invention are those which have a melting point above about 150 C. (the lowest temperature at which the acid treated particulate graphite expands). For example, iron, steel, copper, brass, aluminum, magnesium, zinc, nickel, and the like are suitably cast in accordance with the present invention.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, and are not to be construed to limit, applicants invention.

Example 1 A mold for molten steel was prepared with three risers each having a diameter of about five inches. Molten steel (temperature about 3060" F.) was poured into the mold in quantity suflicient that molten steel was present in each of the three risers. Immediately after pouring, the exposed steel surface in each riser was covered with a weighed amount of an insulating hot-topping material. 'To measure insulating efiiciency, a thermocouple was positioned one-half inch above the hot-topping material in each riser.

Insulating material applied in the first riser was 279 grams of Ferrux, a commercially available noncarbonaceous hot-topping composition containing exothermic reaction materials. This insulating material had a bulk thermal conductivity value of about 1.4 B.t.u./ hour/ F./inch/ft. at an apparent bull: density of about 83 lb./ft. at a temperature range of 465 to646 F. Maximum temperature sensed by the thermocouple in the riser containing this hot-topping composition was 450 C.

In the second riser, grams of Sheaffers, a carbouaceous insulative composition having a bulk thermal conductivity value of 0.96 B.t.u./hour/ F./inch/ft. in a temperature range of 465 to 646 F. at an apparent bulk density of 43.3 lb./ft. was applied. Maximum temperature sensed by the thermocouple above this material was 806 C.

A heat expandable flake graphite (122 grams) prepared as hereinbefore described was applied to the molten steel surface in the third riser (about 10 gm./in. of exposed steel surface). On contact with the hot molten steel, the graphite expanded to form a light weight insulating material. Maximum temperature sensed by the thermocouple above this hot-topping composition was only 216 C. A separate sample of this same graphite when expanded at about the same temperature and at an apparent bulk density of about 1.1 lb./it. was found to have a bulk thermal conductivity value of about 1.2 B.t.u./hour/ F./inch/ft. at a temperature range of 465 to 646 F. In comparing the thermal conductivity of the expanded graphite with the conventional carbonaceous and non-carbonaceous insulating hot-topping materials, it is seen that the insulating power of the expanded graphite was equivalent to that of the commercial riser compounds at A and f of the apparent bulk density of the respective riser compounds.

Example 2 For another comparison employing a mold having three risers of the same size and a molten steel melt, 115 grams of Sheatlers was applied to the exposed steel surface in one riser, 40 grams of expandable graphite was applied to the steel in another riser (about 2 grn./in. of exposed steel surface) and 36 grams of a previously expanded graphite was applied to the steel in the third riser. Temperature readings taken with a thermocouple one-half inch above the insulating compounds indicated maximum temperatures over the Shealiers of 850 C. over the expandable graphite of only 750 C., and over the previously expanded graphite of only 650 C.

Thus a much smaller weight of expandable graphite or expanded graphite provides better insulation than the commercial product.

Example 3 In a manner similar to that described in Examples 1 and 2, beneficial results were also obtained when the expandable flake graphite or the already expanded graphite was applied to the surface of magnesium metal collection wells in electrolytic magnesium cells and aluminum metal melting pots.

Various modifications may be made iu'the presentinvention without departing from the spirit or scope thereof, and it is to be understood that we limit ourselves only as defined in the appended claims.

We claim:

1. In a method of hot-topping a molten metal by covering said molten metal with a refractory, particulate material, the improvement which comprises: covering the exposed surface of said molten metal with a layer of an acid-treated particulate graphite, said metal being at a minimum temperature of about C. and said graphite being further characterized as one which expands into vermicular particles upon being heated at a minimum temperature of about 150 C.

2. The method in accordance with claim 1, including the step of subjecting said acid-treated particulate graphite to a temperature of at least about 150 C. thereby rendering said graphite in an expanded state prior to covering the surface of said molten metal with said graphite.

3. In a method of hot-topping molten metals by covering said molten metals with a refractory, particulate material, the improvement which comprises: covering the exposed surface of said molten metal with a layer of a heat expandable graphite in an amount of from about 1 to about 10 grams of said expandable graphite per square inch of molten metal surface, While maintaining the temwith a layer of said expandable graphite in an amount perature of said molten metal at a minimum of about of from about 1 to about 10 grams of expandable 150 C. graphite per square inch of molten metal surf-ace, 4. In a method of hot-topping molten metals by covwhile maintaining the temperature of said molten ering said molten metals with a refractory, particulate 5 metal at a minimum of about 150 C. material, the improvement which comprises:

(a) providing a supply of a particulate natural graphite; No references cited- (b) acid treating said crystalline graphite and rendering said graphite into a heat expandable form; and SPENCER OVERHQLSER Exammer- (c) covering the exposed surface of a molten metal 10 V. K. RISING, Assistant Examiner. 

1. IN A METHOD OF HOT-TOPPIN A MOLTEN METAL BY COVERING SAID MOLTEN METAL WITH A REFRACTORY, PARTICULATE MATERIAL, THE IMPROVEMENT WHICH COMPRISES: COVERING THE EXPOSED SURFACE OF SAID MOLTEN METAL WITH A LAYER OF AN ACID-TREATED PARTICULATE GRAPHITE, SAID METAL BEING AT A MINIMUM TEMPERATURE OF ABOUT 150*C. AND SAID GRAPHITE BEING FURTHER CHARACTERIZED AS ONE WHICH EXPANDS INTO VERMICULAR PARTICLES UPON BEING HEATED AT A MINIMUM TEMPERATURE OF ABOUT 150*C. 